1. Field of the Invention
The present invention generally relates to implantable medical devices, such as cardiac pacemakers, and in particular to the monitoring of parameters useful for the prediction of atrial fibrillation.
2. Description of the Prior Art
Atrial fibrillation (AF) is a cardiac arrhythmia, i.e. a an altered electrical activity (irregular or faster or slower than normal) of the heart, that involves the atria. AF may be detected as irregularities when taking a pulse. AF is the most common form of arrhythmia affecting approximately 3-5% of people over 65 and 8% of people over 80. For example, there are about 2.2 million cases in the U.S. yearly.
In AF, the electrical impulses that are normally generated by the sinoatrial node (the sinus node) are replaced by disorganized activity in the atria, leading to irregular conduction of impulses to the ventricles that generate the heartbeat. This results in irregular heartbeats. AF may be continuous (persistent or permanent AF) or alternating between periods of normal heart rhythm (paroxysmal AF). Over time, the natural tendency of AF is to become continuous/chronic. The type of AF considered to be most dangerous is, perhaps a bit surprising, paroxysmal AF because the recurrent onsets and offsets of fibrillation increases the probability of embolization significantly. During an AF episode, the blood is rather stationary in the atria and coagulation forming blood clots may take place. When the activity then returns to normal, the clots are propelled out into the system, potentially causing strokes etc. Further, paroxysmal AF is the type of AF that normally occurs first, i.e. it is rather unusual that a healthy subject immediately enters chronic AF. As the first type of AF to hit a subject usually is the most dangerous one, it is desirable to predict AF before it actually starts.
In US 2005/0234313, a system and a method for determining a correlation between sleep apnea and sudden cardiac death is disclosed. “Sudden cardiac death” (SCD) is defined as death within one hour of the onset of symptoms without a previously-known disease or without symptoms. It is described that SCD may be predicted in a patient being monitored for sleep apnea by acquisition of respiration data, by means of e.g. impedance, and electrocardiogram (ECG) data, to determine the correlation. The methods of the disclosure are non-invasive procedures.
In EP 1384433, a monitor for early detection of an ischemic heart disease of a patient is disclosed. The invention relies on impedance measurements and the detection of a notch in an impedance signal. It is described that a parameter defined as ΔZ1/ΔZ2 may be used to detect an ischemic condition, such as e.g. AF, wherein ΔZ1 denotes the difference between a maximum value of the measured impedance and the impedance value measured in a plateau occurring in the impedance curve after the notch, and ΔZ2 the difference between the impedance value in the plateau and a minimum value of the measured impedance in an impedance decrease following the plateau.
In WO 2007/038861, a device for predicting potential occurrence of atrial fibrillation is disclosed. The device comprises an input unit for receiving at least one heart activity signal, and a processing unit for deriving data regarding a plurality of cardiac parameters and determining a likelihood of occurrence of atrial fibrillation based on a correlation between at least two of the cardiac parameters. The cardiac parameters disclosed are conduction times, i.e. time intervals between the activation of a first and a second electrode.
However, there remains a need within the art for an improved device and method for early and convenient prediction of AF.